In the beginning, the man more than likely didn't tell her he was married. As the relationship grew she found out he was married and from what you've said, she tried to break this off. It seems that this affair has gone on for some time now and it is hard on this man to just walk away and go back to his wife. He has now fallen in love with her and wants to marry her.
Sooner or later the wife is bound to find out about this affair and of course, she is going to be angry. Angry isn't the only feeling she is feeling right now. She is hurt, feels betrayed, and thinks that she has been very stupid for not realizing this affair has been going on for some time now. I think she is feeling kind of stupid and most of all hurt.
I can see how she is angry and is blaming both parties for this betrayal. When the woman found out he was married she should have called it off. I know it is hard and she might love him, but nothing good will come of this in the end. Furthermore, when this woman tried to call off the affair and he refused to let this happen he is as much to blame as she is.
He should have stopped seeing her until he could work out an arrangement with this wife and get a divorce. If the two of them still cared for each other they would wait until the marriage had ended to continue their relationship. I blame them both for not thinking this out and not stop seeing each other when the woman asked him to stop the affair so he didn't hurt his wife.